Radio frequency (RF) power amplifier devices (i.e. power amplifiers and the associated circuitry) are used in a large number of applications, such as cellular telephones and other telecommunication devices. As in many electronic devices, when designing power amplifier devices, various output characteristics and design tradeoffs are considered. For example, the gain, ruggedness, stability, output power, efficiency, and linearity of the devices are taken into account to some extent when designing power amplifier circuitry.
Power amplifiers are fabricated with a base, collector and emitter, for bipolar transistors or with a gate, source and drain, for field-effect transistors. The following description will refer to bipolar transistors for convenience. Generally, the power amplifier device has multiple current paths that exist in parallel or multiple transistors connected in parallel in the monolithic device. However, the current and temperature distributions are different between the different paths. This leads to a phenomenon called current collapse, in which there is an abrupt increase in the collector current as the base-emitter voltage of the power amplifier decreases. Current collapse occurs when a particular path (usually the center path, if multiple current paths exist for a single transistor, or supplying a particular transistor if multiple transistors are present) begins to draw a majority of the collector current because of the non-uniform current distribution, leading to a decrease in the current gain of the power amplifier. Increasing the power density also tends to increase the current collapse problem. Current collapse also occurs when multiple parallel power amplifiers are present in a power amplifier device. If the resistances of the power amplifiers vary due to heating or process differences, for example, collapse occurs in the power amplifier having the lowest resistance. Current collapse degrades the power performance of the power amplifier.
Typically, the distribution of the current in the power amplifiers is controlled using ballasting to avoid the collapse caused by the different characteristics between the current paths or between different transistors (or cells). Ballasting prevents any individual path or cell from taking too much current. More specifically, a ballast resistor having a relatively large resistance is connected between the power amplifier and the bias source. Increasing the resistance of the ballast resistor also aids in isolation between the DC bias and the RF input. However, as the output power increases, the base current increases, thereby increasing the voltage drop across the ballast resistor. Thus, as the ballast resistor increases, the amplifier saturates at lower power levels. Thus, increasing the value of the ballast resistor negatively impacts the maximum output power, efficiency and linearity of the power amplifier.